I can usually tell quickly if the reach to the bars feels too close or too far, compared to my previous setup. I've never paid a dime to a fitter. Some believe in KOP. I'm usually further back. Once set, I measure the setback of the saddle nose to the BB center, so I can replicate it on another bike. Last, I record the saddle tip to center of bars dimension. Saddle height above the rails varies too. It's important to check from the BB center to the area where you contact saddle, but there's always some trial and error involved.